Add more information to various asserts. (#3787)

Author: srawlins

lib/src/model/inheritable.dart

@@ -92,8 +92,12 @@ mixin Inheritable on ContainerMember {
           definingEnclosingContainer.isPublic) {
         assert(
             definingEnclosingContainer == found,
-            'For $element, expected $definingEnclosingContainer to be Object '
-            'or $found, but was neither.');
+            "For '$element' (${element.hashCode}) "
+            "(search element: '$searchElement', ${searchElement?.hashCode}, in "
+            "'${searchElement?.enclosingElement}'), expected "
+            "'$definingEnclosingContainer', which is canonical, to be '$found',"
+            "but was not. Here's the inheritance chain: "
+            '${inheritance.reversed}.');
       }
       if (found != null) {
         return found;
@@ -112,8 +116,19 @@ mixin Inheritable on ContainerMember {
     ];
     var object = packageGraph.specialClasses[SpecialClass.object]!;
 
-    assert(definingEnclosingContainer == object ||
-        inheritance.contains(definingEnclosingContainer));
+    assert(
+        definingEnclosingContainer == object ||
+            inheritance.contains(definingEnclosingContainer), () {
+      var inheritanceDescriptions = inheritance
+          .map((e) =>
+              "'$e' (hashCode: ${e.hashCode}, in library '${e.library}')")
+          .toList();
+      return "Given '$this', on '$enclosingElement' in library '$library', "
+          "the defining enclosing container, '$definingEnclosingContainer' "
+          '(hashCode: ${definingEnclosingContainer.hashCode}, '
+          "in library '${definingEnclosingContainer.library}'), should have "
+          'been Object or contained in: $inheritanceDescriptions';
+    }());
     // Unless the code explicitly extends dart:core's Object, we won't get
     // an entry here.  So add it.
     if (inheritance.last != object) {

test/extensions_test.dart

@@ -70,7 +70,10 @@ class ExtensionMethodsExportTest extends DartdocTestBase {
     var fFunction = package.functions.named('f');
     var reference = getMatchingLinkElement(referenceText, fFunction)
         .commentReferable as ModelElement;
-    expect(identical(reference.canonicalModelElement, expected), isTrue);
+    expect(identical(reference.canonicalModelElement, expected), isTrue,
+        reason: '$expected (${expected.hashCode}) is not '
+            '${reference.canonicalModelElement} '
+            '(${reference.canonicalModelElement.hashCode})');
     expect(expected.isCanonical, isTrue);
     expect(expected.href, endsWith(href));
   }