RFC: Start refactoring to provide more than a single backend.

lib/compiler/implementation/ssa/builder.dart

Index: lib/compiler/implementation/ssa/builder.dart
diff --git a/lib/compiler/implementation/ssa/builder.dart b/lib/compiler/implementation/ssa/builder.dart
index f34da88fee09917f30aefba6cbe8110442d4bed4..aea7a9aa05af039fe9f7bbbae9b28e4357a740f7 100644
--- a/lib/compiler/implementation/ssa/builder.dart
+++ b/lib/compiler/implementation/ssa/builder.dart
@@ -781,7 +781,7 @@ class SsaBuilder implements Visitor {
   SsaBuilder(Compiler compiler, WorkItem work)
     : this.compiler = compiler,
       this.work = work,
-      interceptors = compiler.builder.interceptors,
+      interceptors = compiler.backend.builder.interceptors,
       methodInterceptionEnabled = true,
       elements = work.resolutionTree,
       graph = new HGraph(),
@@ -1460,7 +1460,7 @@ class SsaBuilder implements Visitor {
   }
 
   visitFunctionExpression(FunctionExpression node) {
-    ClosureData nestedClosureData = compiler.builder.closureDataCache[node];
+    ClosureData nestedClosureData = compiler.backend.builder.closureDataCache[node];
     if (nestedClosureData === null) {
       // TODO(floitsch): we can only assume that the reason for not having a
       // closure data here is, because the function is inside an initializer.