Handle (missing) trailing slash in dartdoc

sdk/lib/_internal/compiler/implementation/apiimpl.dart

Index: sdk/lib/_internal/compiler/implementation/apiimpl.dart
diff --git a/sdk/lib/_internal/compiler/implementation/apiimpl.dart b/sdk/lib/_internal/compiler/implementation/apiimpl.dart
index 119e891593df03bc8f2b3646a25551b21ef802c8..0d92a95887cdabcbfdae22d637b8a1c2fd313c2c 100644
--- a/sdk/lib/_internal/compiler/implementation/apiimpl.dart
+++ b/sdk/lib/_internal/compiler/implementation/apiimpl.dart
@@ -34,7 +34,14 @@ class Compiler extends leg.Compiler {
             disallowUnsafeEval: hasOption(options, '--disallow-unsafe-eval'),
             strips: getStrips(options),
             enableConcreteTypeInference:
-              hasOption(options, '--enable-concrete-type-inference'));
+              hasOption(options, '--enable-concrete-type-inference')) {
+    if (!libraryRoot.path.endsWith("/")) {
+      throw new ArgumentError("libraryRoot must end with a /");
+    }
+    if (packageRoot != null && !packageRoot.path.endsWith("/")) {
+      throw new ArgumentError("packageRoot must end with a /");
+    }
+  }
 
   static List<String> getStrips(List<String> options) {
     for (String option in options) {