Expand ResolvedAst to handle synthetic constructors.

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

Index: pkg/compiler/lib/src/serialization/modelz.dart
diff --git a/pkg/compiler/lib/src/serialization/modelz.dart b/pkg/compiler/lib/src/serialization/modelz.dart
index e06cb037efeeba8d0d085cf3b2192ddff6c754dd..b34f787af495ddf812127d047571bba43e0a6172 100644
--- a/pkg/compiler/lib/src/serialization/modelz.dart
+++ b/pkg/compiler/lib/src/serialization/modelz.dart
@@ -588,6 +588,7 @@ abstract class LibraryMemberMixin implements DeserializedElementZ {
 
 abstract class ClassMemberMixin implements DeserializedElementZ {
   ClassElement _class;
+  CompilationUnitElement _compilationUnit;
 
   @override
   Element get enclosingElement => enclosingClass;
@@ -607,7 +608,16 @@ abstract class ClassMemberMixin implements DeserializedElementZ {
   LibraryElement get library => enclosingClass.library;
 
   @override
-  CompilationUnitElement get compilationUnit => enclosingClass.compilationUnit;
+  CompilationUnitElement get compilationUnit {
+    if (_compilationUnit == null) {
+      _compilationUnit =
+          _decoder.getElement(Key.COMPILATION_UNIT, isOptional: true);
+      if (_compilationUnit == null) {
+        _compilationUnit = enclosingClass.compilationUnit;
+      }
+    }
+    return _compilationUnit;
+  }
 }
 
 abstract class InstanceMemberMixin implements DeserializedElementZ {