Introduce BackendImpact to separate enqueueing from data.

pkg/compiler/lib/src/ssa/codegen.dart

Index: pkg/compiler/lib/src/ssa/codegen.dart
diff --git a/pkg/compiler/lib/src/ssa/codegen.dart b/pkg/compiler/lib/src/ssa/codegen.dart
index 7b7596eccd59b5564c0d1fe778a98e709c1ec78f..d0385cf257e3ad807d3dbe26cd9c1e1366b01182 100644
--- a/pkg/compiler/lib/src/ssa/codegen.dart
+++ b/pkg/compiler/lib/src/ssa/codegen.dart
@@ -1697,7 +1697,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
 
     List<js.Expression> arguments = visitArguments(node.inputs, start: 0);
 
-    if (element == backend.getCheckConcurrentModificationError()) {
+    if (element == backend.helpers.checkConcurrentModificationError) {
       // Manually inline the [checkConcurrentModificationError] function.  This
       // function is only called from a for-loop update.  Ideally we would just
       // generate the conditionalcontrol flow in the builder but it adds basic
@@ -1705,7 +1705,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       // confuses loop recognition.
 
       assert(arguments.length == 2);
-      Element throwFunction = backend.getThrowConcurrentModificationError();
+      Element throwFunction = backend.helpers.throwConcurrentModificationError;
       registry.registerStaticInvocation(throwFunction);
 
       // Calling using `(0, #)(#)` instead of `#(#)` separates the property load
@@ -2262,7 +2262,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
             .withSourceInformation(node.sourceInformation));
       }
     } else {
-      Element convertToString = backend.getStringInterpolationHelper();
+      Element convertToString = backend.helpers.stringInterpolationHelper;
       registry.registerStaticUse(convertToString);
       js.Expression jsHelper =
           backend.emitter.staticFunctionAccess(convertToString);