First pass at build dependency graph for barback.

sdk/lib/_internal/pub/lib/src/sdk.dart

Index: sdk/lib/_internal/pub/lib/src/sdk.dart
diff --git a/sdk/lib/_internal/pub/lib/src/sdk.dart b/sdk/lib/_internal/pub/lib/src/sdk.dart
index 0fbe5f966eeb3fb69f4c2272d999ce4868dcfd5b..2335e320ddc079f3266771c4bd1a4f2b49e3e346 100644
--- a/sdk/lib/_internal/pub/lib/src/sdk.dart
+++ b/sdk/lib/_internal/pub/lib/src/sdk.dart
@@ -44,6 +44,11 @@ bool get isBleedingEdge {
 /// Determine the SDK's version number.
 Version _getVersion() {
   var revisionPath = path.join(rootDirectory, "version");
+
+  // If we can't find a version file, assume we're running from the repo and
+  // use a magic bleeding edge version.
+  if (!fileExists(revisionPath)) return new Version.parse("0.1.2+repo");
+
   var version = readTextFile(revisionPath).trim();
 
   // Given a version file like: 0.1.2.0_r17495