Cleanup 1: split parts into their own libraries, remove unused imports

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

Index: pkg/compiler/lib/src/ssa/types_propagation.dart
diff --git a/pkg/compiler/lib/src/ssa/types_propagation.dart b/pkg/compiler/lib/src/ssa/types_propagation.dart
index adda76e42a949252d5284b201a922d3f7e2125ad..fc30ab26a88de36db072146ba6bf6287a1b0a305 100644
--- a/pkg/compiler/lib/src/ssa/types_propagation.dart
+++ b/pkg/compiler/lib/src/ssa/types_propagation.dart
@@ -2,7 +2,15 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of ssa;
+import '../compiler.dart' show Compiler;
+import '../elements/elements.dart';
+import '../js_backend/js_backend.dart';
+import '../types/types.dart';
+import '../universe/selector.dart' show Selector;
+import '../world.dart' show ClassWorld, World;
+
+import 'nodes.dart';
+import 'optimize.dart';
 
 class SsaTypePropagator extends HBaseVisitor implements OptimizationPhase {
   final Map<int, HInstruction> workmap = new Map<int, HInstruction>();