Fix analyzer error in dart2js modelx. We will need to watch for perf regressions.

pkg/compiler/lib/src/elements/modelx.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library elements.modelx;
 
 import '../common.dart';
 import '../common/names.dart' show Identifiers;
 import '../common/resolution.dart' show Resolution, ParsingContext;
 import '../compiler.dart' show Compiler;
@@ skipping 3554 matching lines @@
 
   E get implementation => isPatched ? patch : this;
   E get declaration => isPatch ? origin : this;
 
   /// Applies a patch to this element. This method must be called at most once.
   void applyPatch(PatchMixin<E> patch) {
     assert(this.patch == null, failedAt(this, "Element is patched twice."));
     assert(this.origin == null, failedAt(this, "Origin element is a patch."));
     assert(patch.origin == null, failedAt(patch, "Element is patched twice."));
     assert(patch.patch == null, failedAt(patch, "Patch element is patched."));
-    // ignore: INVALID_ASSIGNMENT
-    this.patch = patch;
-    // ignore: INVALID_ASSIGNMENT
-    patch.origin = this;
+    this.patch = patch as E;
+    patch.origin = this as E;
   }
 }
 
 /// Abstract implementation of the [AstElement] interface.
 abstract class AstElementMixin implements AstElement {
   /// The element whose node defines this element.
   ///
   /// For patched functions the defining element is the patch element found
   /// through [implementation] since its node define the implementation of the
   /// function. For patched classes the defining element is the origin element
@@ skipping 16 matching lines @@
       body = node.asFunctionExpression().body;
     }
     return new ParsedResolvedAst(
         declaration,
         node,
         body,
         definingElement.treeElements,
         definingElement.compilationUnit.script.resourceUri);
   }
 }