[ddc] Cleanup extension type erasure helper

A better API is now available from the kernel nodes directly.

Issue: #49735
Change-Id: I1b4b36e44292cadc3f9d6c495ae93ae20e0db970
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/335944
Reviewed-by: Mark Zhou <[email protected]>
Commit-Queue: Nicholas Shahan <[email protected]>

Author: nshahan

pkg/dev_compiler/lib/src/kernel/compiler.dart

@@ -7233,8 +7233,7 @@ class ProgramCompiler extends ComputeOnceConstantVisitor<js_ast.Expression>
 
   @override
   js_ast.Expression visitIsExpression(IsExpression node) {
-    return _emitIsExpression(
-        node.operand, shallowExtensionTypeErasure(node.type));
+    return _emitIsExpression(node.operand, node.type.extensionTypeErasure);
   }
 
   js_ast.Expression _emitIsExpression(Expression operand, DartType type) {
@@ -7271,7 +7270,7 @@ class ProgramCompiler extends ComputeOnceConstantVisitor<js_ast.Expression>
     var fromExpr = node.operand;
     var jsFrom = _visitExpression(fromExpr);
     if (node.isUnchecked) return jsFrom;
-    var to = shallowExtensionTypeErasure(node.type);
+    var to = node.type.extensionTypeErasure;
     var from = fromExpr.getStaticType(_staticTypeContext);
 
     // If the check was put here by static analysis to ensure soundness, we

pkg/dev_compiler/lib/src/kernel/kernel_helpers.dart

@@ -17,14 +17,6 @@ Never throwUnsupportedAuxiliaryType(AuxiliaryType type) =>
     throw UnsupportedError(
         'Unsupported auxiliary type $type (${type.runtimeType}).');
 
-/// Returns [type] with the immediate type erasure applied.
-///
-/// When [type] is an [ExtensionType] this is equivalent to `type.typeErasure`.
-/// The immediately returned value will not be an [ExtensionType] but it could
-/// still contain other [ExtensionType]s embedded within.
-DartType shallowExtensionTypeErasure(DartType type) =>
-    type is ExtensionType ? type.extensionTypeErasure : type;
-
 Constructor? unnamedConstructor(Class c) =>
     c.constructors.firstWhereOrNull((c) => c.name.text == '');