Reduce usage of .enclosingElement to get enclosing class.

lib/compiler/implementation/ssa/optimize.dart

Index: lib/compiler/implementation/ssa/optimize.dart
diff --git a/lib/compiler/implementation/ssa/optimize.dart b/lib/compiler/implementation/ssa/optimize.dart
index e84c0e907ebf24ccdc52c1f2bb6dbe09596fc028..486d522a0017eba75de513c25e1b4624f6e25e70 100644
--- a/lib/compiler/implementation/ssa/optimize.dart
+++ b/lib/compiler/implementation/ssa/optimize.dart
@@ -1191,7 +1191,7 @@ class BaseRecompilationVisitor extends HBaseVisitor {
     if (other != null &&
         other.isConstantNumber() &&
         field.element != null &&
-        field.element.enclosingElement.isClass()) {
+        field.element.isMember()) {
       // Calculate the field type from the information available.  If
       // we have type information for the field and it contains NUMBER
       // we use it as a candidate for recompilation.