Build new IR for functions returning a constant

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
index 02e78b7fce8abee9707e54097758e1a4a53054b1..79c84c48945d7d1fb723204cc103ddc6c48a02c6 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -163,6 +163,7 @@ class FunctionInlineCache {
 
 class JavaScriptBackend extends Backend {
   SsaBuilderTask builder;
+  SsaFromIrBuilderTask fromIrBuilder;
   SsaOptimizerTask optimizer;
   SsaCodeGeneratorTask generator;
   CodeEmitterTask emitter;
@@ -419,6 +420,7 @@ class JavaScriptBackend extends Backend {
         super(compiler, JAVA_SCRIPT_CONSTANT_SYSTEM) {
     emitter = new CodeEmitterTask(compiler, namer, generateSourceMap);
     builder = new SsaBuilderTask(this);
+    fromIrBuilder = new SsaFromIrBuilderTask(compiler);
     optimizer = new SsaOptimizerTask(this);
     generator = new SsaCodeGeneratorTask(this);
     typeVariableHandler = new TypeVariableHandler(this);
@@ -1260,8 +1262,9 @@ class JavaScriptBackend extends Backend {
         compiler.enqueuer.codegen.registerStaticUse(getCyclicThrowHelper());
       }
     }
-
-    HGraph graph = builder.build(work);
+    HGraph graph = compiler.irBuilder.hasIr(element)
+        ? fromIrBuilder.build(work)
+        : builder.build(work);
     optimizer.optimize(work, graph);
     jsAst.Expression code = generator.generateCode(work, graph);
     generatedCode[element] = code;