Document file names in deps files.

pkg/front_end/lib/src/fasta/kernel/kernel_target.dart

 // Copyright (c) 2016, the Dart 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 file.
 
 library fasta.kernel_target;
 
 import 'dart:async' show Future;
 
 import 'dart:io' show File, IOSink;
 
@@ skipping 268 matching lines @@
     } on InputError catch (e) {
       return handleInputError(uri, e, isFullProgram: true);
     } catch (e, s) {
       return reportCrash(e, s, loader?.currentUriForCrashReporting);
     }
   }
 
   Future writeDepsFile(Uri output, Uri depsFile,
       {Iterable<Uri> extraDependencies}) async {
     String toRelativeFilePath(Uri uri) {
-      return Uri.parse(relativizeUri(uri)).toFilePath();
+      // Ninja expects to find file names relative to the current working
+      // directory. We've tried making them relative to the deps file, but that
+      // doesn't work for downstream projects. Making them absolute also
+      // doesn't work.
+      //
+      // We can test if it works by running ninja twice, for example:
+      //
+      //     ninja -C xcodebuild/ReleaseX64 runtime_kernel -d explain
+      //     ninja -C xcodebuild/ReleaseX64 runtime_kernel -d explain
+      //
+      // The second time, ninja should say:
+      //
+      //     ninja: Entering directory `xcodebuild/ReleaseX64'
+      //     ninja: no work to do.
+      //
+      // It's broken if it says something like this:
+      //
+      //     ninja explain: expected depfile 'patched_sdk.d' to mention
+      //     'patched_sdk/platform.dill', got
+      //     '/.../xcodebuild/ReleaseX64/patched_sdk/platform.dill'
+      return Uri.parse(relativizeUri(uri, base: Uri.base)).toFilePath();
     }
 
     if (loader.first == null) return null;
     StringBuffer sb = new StringBuffer();
     sb.write(toRelativeFilePath(output));
     sb.write(":");
     Set<String> allDependencies = new Set<String>();
     allDependencies.addAll(loader.getDependencies().map(toRelativeFilePath));
     if (extraDependencies != null) {
       allDependencies.addAll(extraDependencies.map(toRelativeFilePath));
@@ skipping 396 matching lines @@
   }
   for (Constructor constructor in superclass.constructors) {
     if (constructor.name.name.isEmpty) {
       return constructor.function.requiredParameterCount == 0
           ? constructor
           : null;
     }
   }
   return null;
 }