Unskip expression evaluation test (#149253)

This test was skipped in flutter/flutter#148737
because it had became flaky because instances created in expression
evaluation were being collected quickly. The change in
dart-lang/sdk@ffbbdb5
prevents things in the VM Service's ring buffer from being GC'd which
should resolve the issue.

I am unable to reproduce this test flaking locally on latest code.

Fixes flutter/flutter#148704



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packages/flutter_tools/test/integration.shard/expression_evaluation_test.dart

@@ -89,7 +89,7 @@ void batch1() {
     await breakInBuildMethod(flutter);
     await evaluateComplexReturningExpressions(flutter);
     await cleanProject();
-  }, skip: true);  // https://github.com/flutter/flutter/issues/148704
+  });
 }
 
 void batch2() {