Deserialize ResolvedAsts and ResolutionImpacts only when needed for compilation.

pkg/compiler/lib/src/serialization/task.dart

Index: pkg/compiler/lib/src/serialization/task.dart
diff --git a/pkg/compiler/lib/src/serialization/task.dart b/pkg/compiler/lib/src/serialization/task.dart
index bfcd9501c57bd1dc4ac8586d20ed368ad800d415..9f319db39dd63efc7256a09f95a587b3ce71a652 100644
--- a/pkg/compiler/lib/src/serialization/task.dart
+++ b/pkg/compiler/lib/src/serialization/task.dart
@@ -41,6 +41,10 @@ class SerializationTask extends CompilerTask implements LibraryDeserializer {
   // retained, for instance impacts, resolution data etc.
   bool supportSerialization = false;
 
+  /// Set this flag to also deserialize [ResolvedAst]s and [ResolutionImpact]s
+  /// in `resolveOnly` mode. Use this for testing only.
+  bool deserializeCompilationDataForTesting = false;
+
   /// If `true`, deserialized data is supported.
   bool get supportsDeserialization => deserializer != null;
 
@@ -117,9 +121,11 @@ class SerializationTask extends CompilerTask implements LibraryDeserializer {
   void deserializeFromText(Uri sourceUri, String serializedData) {
     measure(() {
       if (deserializer == null) {
-        deserializer = new DeserializerSystemImpl(compiler);
+        deserializer = new ResolutionDeserializerSystem(compiler,
+            deserializeCompilationDataForTesting:
+                deserializeCompilationDataForTesting);
       }
-      DeserializerSystemImpl deserializerImpl = deserializer;
+      ResolutionDeserializerSystem deserializerImpl = deserializer;
       DeserializationContext context = deserializerImpl.deserializationContext;
       Deserializer dataDeserializer = new Deserializer.fromText(
           context, sourceUri, serializedData, const JsonSerializationDecoder());