Reduce use of Element in optimize.dart

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

 // Copyright (c) 2016, 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.
 
 import '../closure.dart';
 import '../common.dart';
 import '../compiler.dart' show Compiler;
 import '../elements/resolution_types.dart';
 import '../elements/elements.dart';
 import '../io/source_information.dart';
@@ skipping 622 matching lines @@
         result = new TypeMask.nonNullSubclass(cls.declaration, closedWorld);
       }
       cachedTypeOfThis = result;
     }
     return result;
   }
 
   Map<Element, TypeMask> cachedTypesOfCapturedVariables =
       new Map<Element, TypeMask>();
 
-  TypeMask getTypeOfCapturedVariable(Element element) {
-    assert(element.isField);
+  TypeMask getTypeOfCapturedVariable(FieldElement element) {
     return cachedTypesOfCapturedVariables.putIfAbsent(element, () {
       return TypeMaskFactory.inferredTypeForElement(
           element, _globalInferenceResults);
     });
   }
 
   /// Variables stored in the current activation. These variables are
   /// being updated in try/catch blocks, and should be
   /// accessed indirectly through [HLocalGet] and [HLocalSet].
   Map<Local, HLocalValue> activationVariables = <Local, HLocalValue>{};
 }
 
 /// A synthetic local variable only used with the SSA graph.
 ///
 /// For instance used for holding return value of function or the exception of a
 /// try-catch statement.
 class SyntheticLocal extends Local {
   final String name;
   final ExecutableElement executableContext;
 
   // Avoid slow Object.hashCode.
   final int hashCode = _nextHashCode = (_nextHashCode + 1).toUnsigned(30);
   static int _nextHashCode = 0;
 
   SyntheticLocal(this.name, this.executableContext);
 
   toString() => 'SyntheticLocal($name)';
 }