Error: Dictionary changed size during iteration
### Describe the bug
I’ve encountered this error twice so far in two days, coincidentally(?) since I upgraded to 0.14.1, and saw it on dbt Cloud. The error is:
`Completed with 1 error and 0 warnings:
dictionary changed size during iteration`
In both cases this happened when doing dbt test --models source:*
(Reference: runs #1683050 and #1678913 in dbt Cloud)

### Screenshots and log output
Backtrace:
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/dbt/node_runners.py", line 196, in safe_run
    result = self.compile_and_execute(manifest, ctx)
  File "/usr/local/lib/python3.6/dist-packages/dbt/node_runners.py", line 134, in compile_and_execute
    ctx.node = self.compile(manifest)
  File "/usr/local/lib/python3.6/dist-packages/dbt/node_runners.py", line 300, in compile
    return compile_node(self.adapter, self.config, self.node, manifest, {})
  File "/usr/local/lib/python3.6/dist-packages/dbt/compilation.py", line 221, in compile_node
    node = compiler.compile_node(node, manifest, extra_context)
  File "/usr/local/lib/python3.6/dist-packages/dbt/compilation.py", line 118, in compile_node
    compiled_node, self.config, manifest)
  File "/usr/local/lib/python3.6/dist-packages/dbt/context/runtime.py", line 150, in generate
    model, runtime_config, manifest, None, Provider())
  File "/usr/local/lib/python3.6/dist-packages/dbt/context/common.py", line 496, in generate
    return generate_model(model, config, manifest, source_config, provider)
  File "/usr/local/lib/python3.6/dist-packages/dbt/context/common.py", line 471, in generate_model
    source_config, provider)
  File "/usr/local/lib/python3.6/dist-packages/dbt/context/common.py", line 408, in generate_base
    "graph": manifest.to_flat_graph(),
  File "/usr/local/lib/python3.6/dist-packages/dbt/contracts/graph/manifest.py", line 235, in to_flat_graph
    k: v.serialize() for k, v in self.nodes.items()
  File "/usr/local/lib/python3.6/dist-packages/dbt/contracts/graph/manifest.py", line 235, in <dictcomp>
    k: v.serialize() for k, v in self.nodes.items()
  File "/usr/local/lib/python3.6/dist-packages/dbt/api/object.py", line 63, in serialize
    return copy.deepcopy(self._contents)
  File "/usr/lib/python3.6/copy.py", line 150, in deepcopy
    y = copier(x, memo)
  File "/usr/lib/python3.6/copy.py", line 239, in _deepcopy_dict
    for key, value in x.items():
RuntimeError: dictionary changed size during iteration

### System information
**Which database are you using dbt with?**
- [ ] postgres
- [ ] redshift
- [ ] bigquery
- [x] snowflake
- [ ] other (specify: ____________)


**The output of `dbt --version`:**
```
installed version: 0.14.1
   latest version: 0.14.1
```

**The operating system you're using:**
The error occured on dbt Cloud

**The output of `python --version`:**

### Additional context
Add any other context about the problem here.

diff --git a/core/dbt/context/common.py b/core/dbt/context/common.py
index bc03eb8f9..6d91d9ead 100644
--- a/core/dbt/context/common.py
+++ b/core/dbt/context/common.py
@@ -405,7 +405,7 @@ def generate_base(model, model_dict, config, manifest, source_config,
         "exceptions": dbt.exceptions.wrapped_exports(model),
         "execute": provider.execute,
         "flags": dbt.flags,
-        "graph": manifest.to_flat_graph(),
+        "graph": manifest.flat_graph,
         "log": log,
         "model": model_dict,
         "modules": get_context_modules(),
diff --git a/core/dbt/contracts/graph/manifest.py b/core/dbt/contracts/graph/manifest.py
index 432564eac..8751b0154 100644
--- a/core/dbt/contracts/graph/manifest.py
+++ b/core/dbt/contracts/graph/manifest.py
@@ -185,7 +185,7 @@ class Manifest(APIObject):
         self.generated_at = generated_at
         self.metadata = metadata
         self.disabled = disabled
-        self._flat_graph = None
+        self.flat_graph = None
         super(Manifest, self).__init__()
 
     @staticmethod
@@ -224,18 +224,17 @@ class Manifest(APIObject):
             'disabled': [v.serialize() for v in self.disabled],
         }
 
-    def to_flat_graph(self):
-        """This function gets called in context.common by each node, so we want
-        to cache it. Make sure you don't call this until you're done with
-        building your manifest!
+    def build_flat_graph(self):
+        """This attribute is used in context.common by each node, so we want to
+        only build it once and avoid any concurrency issues around it.
+        Make sure you don't call this until you're done with building your
+        manifest!
         """
-        if self._flat_graph is None:
-            self._flat_graph = {
-                'nodes': {
-                    k: v.serialize() for k, v in self.nodes.items()
-                },
-            }
-        return self._flat_graph
+        self.flat_graph = {
+            'nodes': {
+                k: v.serialize() for k, v in self.nodes.items()
+            },
+        }
 
     def find_disabled_by_name(self, name, package=None):
         return dbt.utils.find_in_list_by_name(self.disabled, name, package,
diff --git a/core/dbt/task/runnable.py b/core/dbt/task/runnable.py
index c7443d44f..29682a0cc 100644
--- a/core/dbt/task/runnable.py
+++ b/core/dbt/task/runnable.py
@@ -45,6 +45,7 @@ class ManifestTask(ConfiguredTask):
     def _runtime_initialize(self):
         self.manifest = load_manifest(self.config)
         self.linker = compile_manifest(self.config, self.manifest)
+        self.manifest.build_flat_graph()
 
 
 class GraphRunnableTask(ManifestTask):
['core/dbt/contracts/graph/manifest.py', 'core/dbt/task/runnable.py', 'core/dbt/context/common.py']