Use the VM to detect when a precompiled executable is out-of-date.

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

Index: sdk/lib/_internal/pub/lib/src/entrypoint.dart
diff --git a/sdk/lib/_internal/pub/lib/src/entrypoint.dart b/sdk/lib/_internal/pub/lib/src/entrypoint.dart
index c1f0e7d50960e254b1b10581de90999d77065845..fddd024bc91455c2ae15042c14d184d764ea4355 100644
--- a/sdk/lib/_internal/pub/lib/src/entrypoint.dart
+++ b/sdk/lib/_internal/pub/lib/src/entrypoint.dart
@@ -164,6 +164,7 @@ class Entrypoint {
 
     // If the existing executable was compiled with a different SDK, we need to
     // recompile regardless of what changed.
+    // TODO(nweiz): Use the VM to check this when issue 20802 is fixed.
     var sdkMatches = fileExists(sdkVersionPath) &&
         readTextFile(sdkVersionPath) == "${sdk.version}\n";
     if (!sdkMatches) changed = null;