Return empty contents for not existing sources.

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 7054910d2b3eefa4d22966123d17ffc604fae0ff..bba412cdb0acd7cb53053b0a183958aeedb3a378 100644
--- a/pkg/analyzer/lib/src/context/context.dart
+++ b/pkg/analyzer/lib/src/context/context.dart
@@ -1025,6 +1025,12 @@ class AnalysisContextImpl implements InternalAnalysisContext {
     if (!AnalysisEngine.isDartFileName(source.shortName)) {
       return null;
     }
+    try {
+      getContents(source);
+    } catch (exception, stackTrace) {
+      throw new AnalysisException('Could not get contents of $source',
+          new CaughtException(exception, stackTrace));
+    }
     return _computeResult(source, PARSED_UNIT);
   }