Fix filename handling for Windows. (flutter/engine#4482)

* Fix filename handling for Windows.

Bug: https://github.com/flutter/flutter/issues/13708

* Fix tyop

* Add final to make analyzer happy

engine/src/flutter/frontend_server/lib/server.dart

@@ -128,7 +128,6 @@ class _FrontendCompiler implements CompilerInterface {
   BinaryPrinterFactory printerFactory;
 
   IncrementalKernelGenerator _generator;
-  String _filename;
   String _kernelBinaryFilename;
 
   @override
@@ -137,7 +136,7 @@ class _FrontendCompiler implements CompilerInterface {
     ArgResults options, {
     IncrementalKernelGenerator generator,
   }) async {
-    _filename = filename;
+    final Uri _filenameUri = Uri.base.resolve(new Uri.file(filename).toString());
     _kernelBinaryFilename = "$filename.dill";
     final String boundaryKey = new Uuid().generateV4();
     _outputStream.writeln("result $boundaryKey");
@@ -157,7 +156,7 @@ class _FrontendCompiler implements CompilerInterface {
       _generator = generator != null
           ? generator
           : await IncrementalKernelGenerator.newInstance(
-              compilerOptions, Uri.base.resolve(_filename),
+              compilerOptions, _filenameUri,
               useMinimalGenerator: true);
       final DeltaProgram deltaProgram =
           await _runWithPrintRedirection(() => _generator.computeDelta());
@@ -170,9 +169,8 @@ class _FrontendCompiler implements CompilerInterface {
           sdkRoot.resolve('platform.dill')
         ];
       }
-      program = await _runWithPrintRedirection(() => compileToKernel(
-          Uri.base.resolve(_filename), compilerOptions,
-          aot: options['aot']));
+      program = await _runWithPrintRedirection(() =>
+          compileToKernel(_filenameUri, compilerOptions, aot: options['aot']));
     }
     if (program != null) {
       final IOSink sink = new File(_kernelBinaryFilename).openWrite();
@@ -219,12 +217,11 @@ class _FrontendCompiler implements CompilerInterface {
   }
 
   Uri _ensureFolderPath(String path) {
-    // This is a URI, not a file path, so the forward slash is correct even
-    // on Windows.
-    if (!path.endsWith('/')) {
-      path = '$path/';
+    String uriPath = new Uri.file(path).toString();
+    if (!uriPath.endsWith('/')) {
+      uriPath = '$uriPath/';
     }
-    return Uri.base.resolve(path);
+    return Uri.base.resolve(uriPath);
   }
 
   /// Runs the given function [f] in a Zone that redirects all prints into