Add ContentCache.getExists() and use it in AnalysisContextImpl.exists().

pkg/analyzer/lib/src/generated/source.dart

Index: pkg/analyzer/lib/src/generated/source.dart
diff --git a/pkg/analyzer/lib/src/generated/source.dart b/pkg/analyzer/lib/src/generated/source.dart
index e4ceae7d5e77be2734924ba666dab8535b1581e9..1696bb926c3ce2a4e63c8796dd585a9f0dd770bf 100644
--- a/pkg/analyzer/lib/src/generated/source.dart
+++ b/pkg/analyzer/lib/src/generated/source.dart
@@ -66,6 +66,17 @@ class ContentCache {
   String getContents(Source source) => _contentMap[source.fullName];
 
   /**
+   * Return `true` if the given [source] exists, `false` if it does not exist,
+   * or `null` if this cache does not override existence of the source.

[Paul Berry, 2017/01/04 18:28:57]

It looks like you are trying to plan ahead to a possible future where we might
want the content cache to be able to represent a deletion (i.e. the source
actually exists on disk but the content cache wants to pretend it doesn't).  But
this possible future doesn't exist yet (which is why the code below always
returns either `true` or `null`.

I find this really confusing, and it's hard to test well because we can't easily
write unit tests to exercise a feature that doesn't exist yet.

My recommendation: change this code so that it returns `true` if the given
source is overridden in the content cache, and `false` if it is not overridden. 
Then the implementation just becomes `return
_contentMap.containsKey(source.fullName);` and the caller can be changed the way
I mentioned earlier.

[scheglov, 2017/01/04 20:07:27]

I'm trying to extend ContentCache to fully control AnalysisContext's view of the
world.
So, we can provide a view of the world in the new analysis driver.
We don't want an AnalysisContext to talk to any Source directly.
So, we cannot do contentCache.getExists() || source.exists().

We already have this model for getContents() - it returns a value, or null as "I
don't know".

+   *
+   * <b>Note:</b> This method is not intended to be used except by
+   * [AnalysisContext.exists].
+   */
+  bool getExists(Source source) {
+    return _contentMap.containsKey(source.fullName) ? true : null;

[Brian Wilkerson, 2017/01/04 18:05:10]

Did you intend to return `null` rather than `false`?

+  }
+
+  /**
    * Return the modification stamp of the given [source], or `null` if this
    * cache does not override the contents of the source.
    *