Fix Material docs (#6781)

...by reverting us to an earlier dartdoc package.

Also I added a sanity test to make sure that we don't regress this again.

dev/bots/docs.sh

@@ -2,13 +2,13 @@
 set -e
 
 # Install dartdoc.
-pub global activate dartdoc 0.9.7+6
-
-# Generate flutter docs into dev/docs/doc/api/.
-(cd dev/tools; pub get)
+# Versions after 0.9.7+1 suffer from https://github.com/dart-lang/dartdoc/issues/1236
+# so are we pinned to this old version until that bug is fixed.
+pub global activate dartdoc 0.9.7+1
 
 # This script generates a unified doc set, and creates
 # a custom index.html, placing everything into dev/docs/doc
+(cd dev/tools; pub get)
 FLUTTER_ROOT=$PWD dart dev/tools/dartdoc.dart
 
 # Ensure google webmaster tools can verify our site.

dev/tools/dartdoc.dart

@@ -82,9 +82,24 @@ dependencies:
   if (exitCode != 0)
     exit(exitCode);
 
+  sanityCheckDocs();
+
   createIndexAndCleanup();
 }
 
+void sanityCheckDocs() {
+  List<String> canaries = <String>[
+    '$kDocRoot/api/material/Material-class.html',
+    '$kDocRoot/api/material/Tooltip-class.html',
+    '$kDocRoot/api/widgets/Widget-class.html',
+    '$kDocRoot/api/dart-ui/Canvas-class.html',
+  ];
+  for (String canary in canaries) {
+    if (!new File(canary).existsSync())
+      throw new Exception('Missing "$canary", which probably means the documentation failed to build correctly.');
+  }
+}
+
 /// Creates a custom index.html because we try to maintain old
 /// paths. Cleanup unused index.html files no longer needed.
 void createIndexAndCleanup() {