Stub implementation of patch invariants for the patch refactoring.

lib/compiler/implementation/typechecker.dart

Index: lib/compiler/implementation/typechecker.dart
diff --git a/lib/compiler/implementation/typechecker.dart b/lib/compiler/implementation/typechecker.dart
index 50d34a1fc8b31635606cc8e5e4c877f9b420782a..c34ea42966ea076133ada47ab8eda3895ad6cb9f 100644
--- a/lib/compiler/implementation/typechecker.dart
+++ b/lib/compiler/implementation/typechecker.dart
@@ -26,6 +26,7 @@ class TypeCheckerTask extends CompilerTask {
 
 abstract class DartType implements Hashable {
   abstract SourceString get name;
+  /// Invariant: [element] must be a declaration element.
   abstract Element get element;
 
   /**
@@ -153,7 +154,9 @@ class FunctionType implements DartType {
   Link<DartType> parameterTypes;
 
   FunctionType(DartType this.returnType, Link<DartType> this.parameterTypes,
-               Element this.element);
+               Element this.element) {
+    assert(element == null || element.isDeclaration);
+  }
 
   DartType unalias(Compiler compiler) => this;