Implement deps files in compile_platform.dart.

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

Index: pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
index 5ab3f8f719fc751bc51c98a6084ac87e8d5a8b6b..e13920775982b0b76f2cc1b6ec6089f3a2169022 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
@@ -68,6 +68,8 @@ import '../util/relativize.dart' show relativizeUri;
 
 import '../compiler_context.dart' show CompilerContext;
 
+import '../util/relativize.dart' show relativizeUri;
+
 import 'kernel_builder.dart'
     show
         Builder,
@@ -261,6 +263,20 @@ class KernelTarget extends TargetImplementation {
     }
   }
 
+  Future writeDepsFile(Uri output, Uri depsFile) async {
+    if (loader.first == null) return null;
+    StringBuffer sb = new StringBuffer();
+    Uri base = depsFile.resolve(".");
+    sb.write(Uri.parse(relativizeUri(output, base: base)).toFilePath());
+    sb.write(":");
+    for (Uri dependency in loader.getDependencies()) {
+      sb.write(" ");
+      sb.write(Uri.parse(relativizeUri(dependency, base: base)).toFilePath());
+    }
+    sb.writeln();
+    await new File.fromUri(depsFile).writeAsString("$sb");
+  }
+
   Program erroneousProgram(bool isFullProgram) {
     Uri uri = loader.first?.uri ?? Uri.parse("error:error");
     Uri fileUri = loader.first?.fileUri ?? uri;