Refactor implementation to address review comments and API design suggestions from Luke.

pkg/fletchc/lib/src/fletch_compiler.dart

+// Copyright (c) 2015, the Fletch project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE.md file.
+
+library fletchc.fletch_compiler;
+
+import 'package:compiler/compiler.dart' as api;
+
+import 'package:compiler/src/apiimpl.dart' as apiimpl;
+
+import 'package:sharedfrontend/elements.dart' as elements;
+
+import 'fletch_context.dart';
+
+part 'fletch_compiler_hack.dart';
+
+const EXTRA_DART2JS_OPTIONS = const <String>[
+    // TODO(ahe): This doesn't completely disable type inference. Investigate.
+    '--disable-type-inference',
+    '--output-type=dart',
+];
+
+class FletchCompiler extends FletchCompilerHack {
+  FletchContext internalContext;
+
+  FletchCompiler(
+      api.CompilerInputProvider provider,
+      api.CompilerOutputProvider outputProvider,
+      api.DiagnosticHandler handler,
+      Uri libraryRoot,
+      Uri packageRoot,
+      List<String> options,
+      Map<String, dynamic> environment)
+      : super(
+          provider, outputProvider, handler, libraryRoot, packageRoot,
+          EXTRA_DART2JS_OPTIONS.toList()..addAll(options), environment);
+
+  FletchContext get context {
+    if (internalContext == null) {
+      internalContext = new FletchContext(this);
+    }
+    return internalContext;
+  }
+
+  void computeMain() {
+    if (mainApp == null) return;
+
+    mainFunction = mainApp.findLocal("_entry");
+  }
+
+  void onLibraryCreated(elements.LibraryElement library) {
+    // TODO(ahe): Remove this.
+    library.canUseNative = true;
+    super.onLibraryCreated(library);
+  }
+}