Support top-level field declaration and assignment in analyzer2dart.

pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
index d5bad5dbef60864dcb65d8fc50635e5cd30f4be0..d4620a1568a93d31991611960c2e9e050da4330f 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
@@ -147,6 +147,9 @@ abstract class IrBuilderMixin<N> {
     return result;
   }
 
+  /// `true` if an [IrBuilder] is currently set up.
+  bool get hasIrBuilder => _irBuilder != null;
+
   /// The current builder.
   IrBuilder get irBuilder {
     assert(_irBuilder != null);
@@ -534,18 +537,23 @@ class IrBuilder {
   }
 
   /// Create a [ir.FieldDefinition] for the current [Element] using [_root] as
-  /// the body.
-  ir.FieldDefinition makeFieldDefinition() {
-    return new ir.FieldDefinition(state.currentElement,
-                                  state.returnContinuation,
-                                  _root);
+  /// the body using [initializer] as the initial value.
+  ir.FieldDefinition makeFieldDefinition(ir.Primitive initializer) {
+    if (initializer == null) {
+      return new ir.FieldDefinition.withoutInitializer(state.currentElement);
+    } else {
+      buildReturn(initializer);
+      return new ir.FieldDefinition(state.currentElement,
+                                    state.returnContinuation,
+                                    _root);
+    }
   }
 
   /// Create a [ir.FunctionDefinition] for [element] using [_root] as the body.
   ///
   /// Parameters must be created before the construction of the body using
   /// [createParameter].
-  ir.FunctionDefinition buildFunctionDefinition(
+  ir.FunctionDefinition makeFunctionDefinition(
       List<ConstantExpression> defaults) {
     FunctionElement element = state.currentElement;
     if (element.isAbstract || element.isExternal) {