Add option to gen_snapshot for creating a Makefile describing a snapshot's dependencies.

runtime/bin/vmservice/loader.dart

Index: runtime/bin/vmservice/loader.dart
diff --git a/runtime/bin/vmservice/loader.dart b/runtime/bin/vmservice/loader.dart
index a72e735df2cbe3f27bba5d090a49fad0c0dc42db..7710fe008ac0cca3f17472a5b5475d5f0015d30a 100644
--- a/runtime/bin/vmservice/loader.dart
+++ b/runtime/bin/vmservice/loader.dart
@@ -953,7 +953,10 @@ const _Dart_kResourceLoad = 5;         // Resource class support.
 const _Dart_kGetPackageRootUri = 6;    // Uri of the packages/ directory.
 const _Dart_kGetPackageConfigUri = 7;  // Uri of the .packages file.
 const _Dart_kResolvePackageUri = 8;    // Resolve a package: uri.
+
+// Extra requests. Keep these in sync between loader.dart and loader.cc.
 const _Dart_kImportExtension = 9;      // Import a dart-ext: file.
+const _Dart_kResolveAsFilePath = 10;   // Resolve uri to file path.
 
 // External entry point for loader requests.
 _processLoadRequest(request) {
@@ -1115,6 +1118,36 @@ _processLoadRequest(request) {
         break;
       }
     break;
+    case _Dart_kResolveAsFilePath:
+      String uri = request[4];
+      Uri resolvedUri = Uri.parse(uri);
+      try {
+        if (resolvedUri.scheme == 'package') {
+          resolvedUri = loaderState._resolvePackageUri(resolvedUri);
+        }
+        if (resolvedUri.scheme == '' || resolvedUri.scheme == 'file') {
+          resolvedUri = loaderState._workingDirectory.resolveUri(resolvedUri);
+          var msg = new List(5);
+          msg[0] = tag;
+          msg[1] = uri;
+          msg[2] = resolvedUri.toString();
+          msg[3] = null;
+          msg[4] = resolvedUri.toFilePath();
+          sp.send(msg);
+        } else {
+          throw "Cannot resolve scheme (${resolvedUri.scheme}) to file path"
+                " for $resolvedUri";
+        }
+      } catch (e) {
+        var msg = new List(5);
+        msg[0] = -tag;
+        msg[1] = uri;
+        msg[2] = resolvedUri.toString();
+        msg[3] = null;
+        msg[4] = e.toString();
+        sp.send(msg);
+      }
+    break;
     default:
       _log('Unknown loader request tag=$tag from $isolateId');
   }