dart2js cps: Hoist loop-invariant branches from loop entry.

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

Index: pkg/compiler/lib/src/cps_ir/cps_fragment.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_fragment.dart b/pkg/compiler/lib/src/cps_ir/cps_fragment.dart
index cc06b498c2f77e038b62b7ce2b03870a4900de10..7e62fa5f7066885056dc925b13754bfa4039e0ee 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_fragment.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_fragment.dart
@@ -189,31 +189,39 @@ class CpsFragment {
 
   /// Branch on [condition].
   ///
-  /// Returns a new fragment for the 'then' branch.
+  /// Returns a new fragment for the 'then' branch, or the 'else' branch
+  /// if [negate] is true.
   ///
-  /// The 'else' branch becomes the new hole.
-  CpsFragment ifTruthy(Primitive condition) {
+  /// The other branch becomes the new hole.
+  CpsFragment branch(Primitive condition,
+                     {bool negate: false,
+                      bool strict: false}) {
     Continuation trueCont = new Continuation(<Parameter>[]);
     Continuation falseCont = new Continuation(<Parameter>[]);
     put(new LetCont.two(trueCont, falseCont,
-            new Branch.loose(condition, trueCont, falseCont)));
-    context = falseCont;
-    return new CpsFragment(sourceInformation, trueCont);
+            new Branch(condition, trueCont, falseCont, strict: strict)));
+    if (negate) {
+      context = trueCont;
+      return new CpsFragment(sourceInformation, falseCont);
+    } else {
+      context = falseCont;
+      return new CpsFragment(sourceInformation, trueCont);
+    }
   }
 
   /// Branch on [condition].
   ///
+  /// Returns a new fragment for the 'then' branch.
+  ///
+  /// The 'else' branch becomes the new hole.
+  CpsFragment ifTruthy(Primitive condition) => branch(condition);
+
+  /// Branch on [condition].
+  ///
   /// Returns a new fragment for the 'else' branch.
   ///
   /// The 'then' branch becomes the new hole.
-  CpsFragment ifFalsy(Primitive condition) {
-    Continuation trueCont = new Continuation(<Parameter>[]);
-    Continuation falseCont = new Continuation(<Parameter>[]);
-    put(new LetCont.two(trueCont, falseCont,
-            new Branch.loose(condition, trueCont, falseCont)));
-    context = trueCont;
-    return new CpsFragment(sourceInformation, falseCont);
-  }
+  CpsFragment ifFalsy(Primitive condition) => branch(condition, negate: true);
 
   /// Create a new empty continuation and bind it here.
   ///