Normalize type masks to use the least upper instantiated subclass/type.

pkg/compiler/lib/src/ssa/builder.dart

Index: pkg/compiler/lib/src/ssa/builder.dart
diff --git a/pkg/compiler/lib/src/ssa/builder.dart b/pkg/compiler/lib/src/ssa/builder.dart
index 35cf64c929b681008251d77a87edd86be31f7a14..5e89a0080dc550c283301e662a0b37d87323e350 100644
--- a/pkg/compiler/lib/src/ssa/builder.dart
+++ b/pkg/compiler/lib/src/ssa/builder.dart
@@ -3247,7 +3247,7 @@ class SsaBuilder extends ast.Visitor
     });
 
     TypeMask type =
-        new TypeMask.nonNullExact(coreClasses.functionClass, compiler.world);
+        new TypeMask.nonNullExact(closureClassElement, compiler.world);
     push(new HForeignNew(closureClassElement, type, capturedVariables)
         ..sourceInformation = sourceInformationBuilder.buildCreate(node));
 
@@ -5074,7 +5074,13 @@ class SsaBuilder extends ast.Visitor
             : inferred;
       } else if (element.isGenerativeConstructor) {
         ClassElement cls = element.enclosingClass;
-        return new TypeMask.nonNullExact(cls.thisType.element, compiler.world);
+        if (cls.isAbstract) {
+          // An error will be thrown.
+          return new TypeMask.nonNullEmpty();
+        } else {
+          return new TypeMask.nonNullExact(
+              cls.thisType.element, compiler.world);
+        }
       } else {
         return TypeMaskFactory.inferredReturnTypeForElement(
             originalElement, compiler);