fix(css): when compiling CSS, leave absolute imports alone

Closes #4592

modules_dart/transform/lib/src/transform/common/url_resolver.dart

@@ -59,9 +59,8 @@ Uri toAssetScheme(Uri absoluteUri) {
     return absoluteUri;
   }
   if (absoluteUri.scheme != 'package') {
-    throw new FormatException(
-        'Unsupported URI scheme "${absoluteUri.scheme}" encountered.',
-        absoluteUri);
+    // Pass through URIs with non-package scheme
+    return absoluteUri;
   }
 
   if (absoluteUri.pathSegments.length < 2) {

modules_dart/transform/test/transform/common/url_resolver_tests.dart

@@ -88,9 +88,9 @@ void allTests() {
           .toThrowWith(anInstanceOf: FormatException);
     });
 
-    it('should throw for unsupported schemes', () {
-      expect(() => toAssetScheme(Uri.parse('file:///angular2')))
-          .toThrowWith(anInstanceOf: FormatException);
+    it('should pass through unsupported schemes', () {
+      var uri = 'http://server.com/style.css';
+      expect('${toAssetScheme(Uri.parse(uri))}').toEqual(uri);
     });
 
     it('should throw if passed a null uri', () {

modules_dart/transform/test/transform/stylesheet_compiler/all_tests.dart

@@ -16,6 +16,9 @@ const SIMPLE_CSS = '''
 }
 ''';
 
+const HTTP_IMPORT = 'https://fonts.googleapis.com/css?family=Roboto';
+const CSS_WITH_IMPORT = '@import url(${HTTP_IMPORT});';
+
 main() {
   Html5LibDomAdapter.makeCurrent();
   allTests();
@@ -60,6 +63,20 @@ allTests() {
     expect(transform.outputs[1].id.toString())
         .toEqual('somepackage|lib/style.css.shim.dart');
   });
+
+  it('should compile stylesheets with imports', () async {
+    var cssFile = new Asset.fromString(
+        new AssetId('somepackage', 'lib/style.css'), CSS_WITH_IMPORT);
+    var transform = new FakeTransform()..primaryInput = cssFile;
+    await subject.apply(transform);
+    expect(transform.outputs.length).toBe(2);
+    expect(transform.outputs[0].id.toString())
+        .toEqual('somepackage|lib/style.css.dart');
+    expect(transform.outputs[1].id.toString())
+        .toEqual('somepackage|lib/style.css.shim.dart');
+    expect(await transform.outputs[0].readAsString()).toContain(HTTP_IMPORT);
+    expect(await transform.outputs[1].readAsString()).toContain(HTTP_IMPORT);
+  });
 }
 
 @proxy