Reland "[cfe] Run suite in together in update_expectations.dart"

This speeds up update_expectations.dart by avoid to restart
the expectation testing for each suite.

Change-Id: If0a4562f92f106c1c04bd22a81eb28e52591c4b1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/343220
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

pkg/testing/lib/src/chain.dart

@@ -72,7 +72,7 @@ class Chain extends Suite {
   void writeClosureOn(StringSink sink) {
     sink.write("await runChain(");
     sink.write(name);
-    sink.writeln(".createContext, environment, selectors, r'''");
+    sink.writeln(".createContext, {...environment}, selectors, r'''");
     const String jsonExtraIndent = "    ";
     sink.write(jsonExtraIndent);
     sink.writeAll(splitLines(JsonEncoder.withIndent("  ").convert(this)),