dart2js cps: Support 'on T' clauses without a 'catch'.

pkg/compiler/lib/src/cps_ir/cps_ir_builder.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 dart2js.ir_builder;
 
 import '../compile_time_constants.dart' show BackendConstantEnvironment;
 import '../constants/constant_system.dart';
 import '../constants/values.dart' show ConstantValue, PrimitiveConstantValue;
 import '../dart_types.dart';
@@ skipping 1811 matching lines @@
           break;
         }
         if (traceVariable == null) {
           traceVariable = info.stackTraceVariable;
         }
       }
       ir.Parameter traceParameter = new ir.Parameter(traceVariable);
 
       ir.Expression buildCatchClause(CatchClauseInfo clause) {
         IrBuilder clauseBuilder = builder.makeDelimitedBuilder();
-        clauseBuilder.declareLocalVariable(clause.exceptionVariable,
-            initialValue: exceptionParameter);
+        if (clause.exceptionVariable != null) {
+          clauseBuilder.declareLocalVariable(clause.exceptionVariable,
+              initialValue: exceptionParameter);
+        }
         if (clause.stackTraceVariable != null) {
           clauseBuilder.declareLocalVariable(clause.stackTraceVariable,
               initialValue: traceParameter);
         }
         clause.buildCatchBlock(clauseBuilder);
         if (clauseBuilder.isOpen) clauseBuilder.jumpTo(join);
         return clauseBuilder._root;
       }
 
       // Expand multiple catch clauses into an explicit if/then/else.  Iterate
@@ skipping 919 matching lines @@
 }
 
 class SwitchCaseInfo {
   final List<ir.Primitive> constants = <ir.Primitive>[];
   final SubbuildFunction buildBody;
 
   SwitchCaseInfo(this.buildBody);
 
   void addConstant(ir.Primitive constant) => constants.add(constant);
 }