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

pkg/analyzer/lib/src/context/context.dart

Index: pkg/analyzer/lib/src/context/context.dart
diff --git a/pkg/analyzer/lib/src/context/context.dart b/pkg/analyzer/lib/src/context/context.dart
index 4f055910a64168ea74047283ffc56ab438a1dcbe..c2c2ec0c47f0a7a1d3149c12a4b5e6793a061831 100644
--- a/pkg/analyzer/lib/src/context/context.dart
+++ b/pkg/analyzer/lib/src/context/context.dart
@@ -810,8 +810,9 @@ class AnalysisContextImpl implements InternalAnalysisContext {
     if (source == null) {
       return false;
     }
-    if (_contentCache.getContents(source) != null) {
-      return true;
+    bool overriddenExists = _contentCache.getExists(source);
+    if (overriddenExists != null) {

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

Assuming you accept my proposal below, I think lines 813 through 817 can be
replaced with:

return _contentCache.getExists(source) || source.exists();

+      return overriddenExists;
     }
     return source.exists();
   }