tree-ir: Treat getInterceptor and identical as effectively constant.

pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart

Index: pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
diff --git a/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart b/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
index 8227e7f62a7c47388cd15533175cea0c4af83c5b..2d538659ab78b9e24b48b37491313081300ad749 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
@@ -173,6 +173,14 @@ class InvokeStatic extends Expression implements Invoke {
   final Selector selector;
   final SourceInformation sourceInformation;
 
+  /// True if the [target] is known not to diverge or read or write any
+  /// mutable state.
+  ///
+  /// This is set for calls to `getInterceptor` and `identical` to indicate
+  /// that they can be safely be moved across an impure expression
+  /// (assuming the [arguments] are not affected by the impure expression).
+  bool isEffectivelyConstant = false;
+
   InvokeStatic(this.target, this.selector, this.arguments,
                {this.sourceInformation});