[ddc] Erase extension types in assertions

Ensures the same optimizations to expressions statically typed as
extension types.

Issue: #49735
Change-Id: Id9b944d3f716faf38bb4770eb9b90ccc708056ce
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/342104
Reviewed-by: Mark Zhou <[email protected]>

Author: nshahan

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

@@ -4699,14 +4699,16 @@ class ProgramCompiler extends ComputeOnceConstantVisitor<js_ast.Expression>
   js_ast.Statement visitAssertStatement(AssertStatement node) {
     if (!_options.enableAsserts) return js_ast.EmptyStatement();
     var condition = node.condition;
-    var conditionType = condition.getStaticType(_staticTypeContext);
+    var conditionType =
+        condition.getStaticType(_staticTypeContext).extensionTypeErasure;
     var jsCondition = _visitExpression(condition);
 
     if (conditionType != _coreTypes.boolLegacyRawType &&
         conditionType != _coreTypes.boolNullableRawType &&
         conditionType != _coreTypes.boolNonNullableRawType) {
       jsCondition = runtimeCall('dtest(#)', [jsCondition]);
     } else if (isNullable(condition)) {
+      // TODO(nshahan): Is this branch even reachable in null safe code?
       jsCondition = runtimeCall('test(#)', [jsCondition]);
     }