Fix some warnings.

lib/compiler/implementation/ssa/codegen.dart

 // Copyright (c) 2012, 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.
 
 class SsaCodeGeneratorTask extends CompilerTask {
 
   final JavaScriptBackend backend;
 
   SsaCodeGeneratorTask(JavaScriptBackend backend)
       : this.backend = backend,
@@ skipping 58 matching lines @@
         if (block.last is HReturn) {
           backend.registerReturnType(work.element, types[block.last.inputs[0]]);
         } else {
           backend.registerReturnType(work.element, HType.NULL);
         }
       });
       compiler.tracer.traceGraph("codegen", graph);
       Map<Element, String> parameterNames = getParameterNames(work);
       // Use [work.element] to ensure that the parameter element come from
       // the declaration.
-      work.element.computeSignature(compiler).forEachParameter((element) {
+      FunctionElement function = work.element;
+      function.computeSignature(compiler).forEachParameter((element) {
         compiler.enqueuer.codegen.addToWorkList(element);
       });
       List<js.Parameter> parameters = <js.Parameter>[];
       parameterNames.forEach((element, name) {
         parameters.add(new js.Parameter(name));
       });
       addTypeParameters(work.element, parameters, parameterNames);
       String parametersString = Strings.join(parameterNames.getValues(), ", ");
       SsaOptimizedCodeGenerator codegen = new SsaOptimizedCodeGenerator(
           backend, work, parameters, parameterNames);
@@ skipping 81 matching lines @@
       parameterNames[element] = function.isNative()
           ? element.name.slowToString()
           : JsNames.getValid('${element.name.slowToString()}');
     });
     return parameterNames;
   }
 }
 
 typedef void ElementAction(Element element);
 
-class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
+abstract class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
   /**
    * Returned by [expressionType] to tell how code can be generated for
    * a subgraph.
    * - [TYPE_STATEMENT] means that the graph must be generated as a statement,
    * which is always possible.
    * - [TYPE_EXPRESSION] means that the graph can be generated as an expression,
    * or possibly several comma-separated expressions.
    * - [TYPE_DECLARATION] means that the graph can be generated as an
    * expression, and that it only generates expressions of the form
    *   variable = expression
@@ skipping 1617 matching lines @@
         "==" : "!=",
         "!=" : "==",
         "===": "!==",
         "!==": "===",
         "<"  : ">=",
         "<=" : ">",
         ">"  : "<=",
         ">=" : "<"
       };
       HRelational relational = input;
+      
       visitInvokeBinary(input,
                         inverseOperator[relational.operation.name.stringValue]);
     } else {
       use(input);
       push(new js.Prefix("!", pop()));
     }
   }
 
   visitParameterValue(HParameterValue node) => visitLocalValue(node);
 
@@ skipping 612 matching lines @@
 
       SourceString helper;
       if (node.isBooleanConversionCheck) {
         helper = const SourceString('boolConversionCheck');
       } else {
         helper = backend.getCheckedModeHelper(type);
         if (node.isCastTypeCheck) {
           helper = castNames[helper.stringValue];
         }
       }
-      Element helperElement = compiler.findHelper(helper);
+      FunctionElement helperElement = compiler.findHelper(helper);
       world.registerStaticUse(helperElement);
       List<js.Expression> arguments = <js.Expression>[];
       use(node.checkedInput);
       arguments.add(pop());
       if (helperElement.computeSignature(compiler).parameterCount != 1) {
         String additionalArgument = backend.namer.operatorIs(element);
         arguments.add(new js.LiteralString("'$additionalArgument'"));
       }
       String helperName = backend.namer.isolateAccess(helperElement);
       push(new js.Call(new js.VariableUse(helperName), arguments));
@@ skipping 520 matching lines @@
   if (leftType.canBeNull() && rightType.canBeNull()) {
     if (left.isConstantNull() || right.isConstantNull() ||
         (leftType.isPrimitive() && leftType == rightType)) {
       return '==';
     }
     return null;
   } else {
     return '===';
   }
 }