Try to not use incomplete Dart types in HType.

sdk/lib/_internal/compiler/implementation/ssa/builder.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/builder.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/builder.dart	(revision 18754)
+++ sdk/lib/_internal/compiler/implementation/ssa/builder.dart	(working copy)
@@ -248,7 +248,9 @@
       Element element = closureData.thisElement;
       ClassElement cls = element.enclosingElement.getEnclosingClass();
       Compiler compiler = builder.compiler;
-      DartType type = cls.computeType(compiler);
+      // Use the raw type because we don't have the type context for the
+      // type parameters.
+      DartType type = cls.rawType;
       if (compiler.world.isUsedAsMixin(cls)) {
         // If the enclosing class is used as a mixin, [:this:] can be
         // of the class that mixins the enclosing class. These two