Ensure supertypes are loaded safely.

dart/lib/compiler/implementation/typechecker.dart

Index: dart/lib/compiler/implementation/typechecker.dart
diff --git a/dart/lib/compiler/implementation/typechecker.dart b/dart/lib/compiler/implementation/typechecker.dart
index 290b68a73e5f58d783a688d94943537d26df3484..8918e9a7a5fed736cfd0fb888b00e08241e9b47a 100644
--- a/dart/lib/compiler/implementation/typechecker.dart
+++ b/dart/lib/compiler/implementation/typechecker.dart
@@ -93,11 +93,11 @@ class InterfaceType implements Type {
 
 class FunctionType implements Type {
   final Element element;
-  final Type returnType;
-  final Link<Type> parameterTypes;
+  Type returnType;
+  Link<Type> parameterTypes;
 
-  const FunctionType(Type this.returnType, Link<Type> this.parameterTypes,
-                     Element this.element);
+  FunctionType(Type this.returnType, Link<Type> this.parameterTypes,
+               Element this.element);
 
   toString() {
     StringBuffer sb = new StringBuffer();
@@ -115,6 +115,13 @@ class FunctionType implements Type {
     parameterTypes.forEach((_) { arity++; });
     return arity;
   }
+
+  void initializeFrom(FunctionType other) {
+    assert(returnType === null);
+    assert(parameterTypes === null);
+    returnType = other.returnType;
+    parameterTypes = other.parameterTypes;
+  }
 }
 
 class Types {