Fix bug in dev-compiler putIfAbsent.

Was adding key to _keymap before calling `ifAbsent`,
so `containsKey` saw it during the `ifAbsent` call.

Modified to allow concurrent modification inthe `ifAbsent` function,
like other platforms already do. Documentation updated to match.

Fixes #47852

CoreLibraryReviewExempt: DDC only change, other than docs.
Bug: https://dartbug.com/47852
Change-Id: Ia289f426b1c9ad37ce89eda5119f9086dc5d39b4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/222200
Commit-Queue: Lasse Nielsen <lrn@google.com>
Reviewed-by: Nate Biggs <natebiggs@google.com>

pkg/expect/lib/expect.dart

@@ -434,6 +434,10 @@ class Expect {
         _fail('Expect.mapEquals(unexpected key: <$key>$msg) fails');
       }
     }
+    if (expectedKeys.length != actualKeys.length) {
+      _failNotEqual(expectedKeys.length, actualKeys.length,
+          "mapEquals", "map.length");
+    }
   }
 
   /// Specialized equality test for strings. When the strings don't match,