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

pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
index ca015802776c66db3a321916dbdbfa618ba81921..a2ae826675c006ca50a737b2649b6fb30c74c964 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
@@ -1829,8 +1829,10 @@ class IrBuilder {
 
       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);