Don't duplicate switch case code if we fall through to the default case.

tests/compiler/dart2js_extra/switch_test.dart

Index: tests/compiler/dart2js_extra/switch_test.dart
diff --git a/tests/compiler/dart2js_extra/switch_test.dart b/tests/compiler/dart2js_extra/switch_test.dart
index 7acade2f3fe8551c601dc191610cd3a9c108b811..5eb51c0db4172f3dae7f4e887d4e30a1514c672d 100644
--- a/tests/compiler/dart2js_extra/switch_test.dart
+++ b/tests/compiler/dart2js_extra/switch_test.dart
@@ -58,6 +58,21 @@ switcher2(val) {
   return x;
 }
 
+var x = 0;
+
+@NoInline()
+switcher3(val) {
+  switch(val) {
+    case 1:
+    default:
+      incrementX();
+  }
+}
+
+incrementX() {
+  x++;
+}
+
 badswitches(val) {
   // Test some badly formed switch bodies.
   // 01 - a label/statement without a following case/default.
@@ -81,5 +96,8 @@ main() {
   Expect.equals(200, switcher2(4));
   Expect.equals(200, switcher2(5));
 
+  switcher3(1);
+  Expect.equals(1, x);
+
   badswitches(42);
 }