Set up a stub pipline for using the new cps-based ir to generate js.

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index fad4298d35af97a8d80cc6d5b4c10ba27292c562..a3a29a966f04cdf9a9600bb2cfa83fd50b5989d0 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -1367,6 +1367,14 @@ abstract class Compiler implements DiagnosticListener {
     });
   }
 
+  bool irEnabled() {
+    // TODO(sigurdm,kmillikin): Support checked-mode checks.
+    return const bool.fromEnvironment('USE_NEW_BACKEND') &&
+        backend is DartBackend &&
+        enableTypeAssertions &&
+        enableConcreteTypeInference;
+  }
+
   void computeMain() {
     if (mainApp == null) return;
 
@@ -1489,8 +1497,10 @@ abstract class Compiler implements DiagnosticListener {
 
     deferredLoadTask.onResolutionComplete(mainFunction);
 
-    log('Building IR...');
-    irBuilder.buildNodes();
+    if (irEnabled()) {
+      log('Building IR...');
+      irBuilder.buildNodes();
+    }
 
     log('Inferring types...');
     typesTask.onResolutionComplete(mainFunction);