Emit warning on import of dart:mirrors.

pkg/compiler/lib/src/util/uri_extras.dart

Index: pkg/compiler/lib/src/util/uri_extras.dart
diff --git a/pkg/compiler/lib/src/util/uri_extras.dart b/pkg/compiler/lib/src/util/uri_extras.dart
index 65a4ed2516112ac27787c9719d5cc45e73642d4c..aa8cdcf7f129271ecbf3c01f5a395248435cc56c 100644
--- a/pkg/compiler/lib/src/util/uri_extras.dart
+++ b/pkg/compiler/lib/src/util/uri_extras.dart
@@ -7,18 +7,20 @@ library uri_extras;
 import 'dart:math';
 
 String relativize(Uri base, Uri uri, bool isWindows) {
-  if (!base.path.startsWith('/')) {
-    // Also throw an exception if [base] or base.path is null.
-    throw new ArgumentError('Expected absolute path: ${base.path}');
-  }
-  if (!uri.path.startsWith('/')) {
-    // Also throw an exception if [uri] or uri.path is null.
-    throw new ArgumentError('Expected absolute path: ${uri.path}');
-  }
   bool equalsNCS(String a, String b) {
     return a.toLowerCase() == b.toLowerCase();
   }
 
+  if (!equalsNCS(base.scheme, uri.scheme) ||
+      equalsNCS(base.scheme, 'dart') ||
+      equalsNCS(base.scheme, 'package')) {
+    return uri.toString();
+  }
+
+  if (!equalsNCS(base.scheme, 'file')) {
+    isWindows = false;
+  }
+
   String normalize(String path) {
     if (isWindows) {
       return path.toLowerCase();
@@ -27,9 +29,7 @@ String relativize(Uri base, Uri uri, bool isWindows) {
     }
   }
 
-  if (equalsNCS(base.scheme, 'file') &&
-      equalsNCS(base.scheme, uri.scheme) &&
-      base.userInfo == uri.userInfo &&
+  if (base.userInfo == uri.userInfo &&
       equalsNCS(base.host, uri.host) &&
       base.port == uri.port &&
       uri.query == "" && uri.fragment == "") {
@@ -37,6 +37,11 @@ String relativize(Uri base, Uri uri, bool isWindows) {
       return uri.path.substring(base.path.lastIndexOf('/') + 1);
     }
 
+    if (!base.path.startsWith('/') ||
+        !uri.path.startsWith('/')) {
+      return uri.toString();
+    }
+
     List<String> uriParts = uri.path.split('/');
     List<String> baseParts = base.path.split('/');
     int common = 0;