Add tests for relative hrefs in doc comments

Author: jdkoren

test/model_test.dart

@@ -1182,6 +1182,21 @@ void main() {
         expect(docsAsHtml,
             contains('<code>ThisIsNotHereNoWay&lt;MyType&gt;</code>'));
       });
+
+      test('leaves relative href resulting in a broken link', () {
+        // Dartdoc does emit a brokenLink warning for this.
+        expect(docsAsHtml,
+            contains('<a href="SubForDocComments/localMethod.html">link</a>'));
+      });
+
+      test('leaves relative href resulting in a working link', () {
+        // Ideally doc comments should not make assumptions about Dartdoc output
+        // files, but unfortunately some do...
+        expect(
+            docsAsHtml,
+            contains(
+                '<a href="../SubForDocComments/localMethod.html">link</a>'));
+      });
     });
 
     test('multi-underscore names in brackets do not become italicized', () {

testing/test_package/lib/fake.dart

@@ -887,6 +887,10 @@ class BaseForDocComments {
   /// Reference containing a type parameter [ExtraSpecialList<Object>]
   ///
   /// Reference to something that doesn't exist containing a type parameter [ThisIsNotHereNoWay<MyType>]
+  ///
+  /// Link to a nonexistent file (erroneously expects base href): [link](SubForDocComments/localMethod.html)
+  ///
+  /// Link to an existing file: [link](../SubForDocComments/localMethod.html)
   String doAwesomeStuff(int value) => null;
 
   void anotherMethod() {}