Add a "missing colon" hint.

Closes dart-lang/yaml#11

R=kathyw@google.com

Review URL: https://codereview.chromium.org//1176653003.

pkgs/yaml/CHANGELOG.md

diff --git a/pkgs/yaml/CHANGELOG.md b/pkgs/yaml/CHANGELOG.md
index b864250..5dcd749 100644
--- a/pkgs/yaml/CHANGELOG.md
+++ b/pkgs/yaml/CHANGELOG.md
@@ -1,3 +1,8 @@
+## 2.1.3
+
+* Add a hint that a colon might be missing when a mapping value is found in the
+  wrong context.
+
 ## 2.1.2
 
 * Fix a crashing bug when parsing block scalars.

pkgs/yaml/lib/src/scanner.dart

diff --git a/pkgs/yaml/lib/src/scanner.dart b/pkgs/yaml/lib/src/scanner.dart
index e1b578e..512d72e 100644
--- a/pkgs/yaml/lib/src/scanner.dart
+++ b/pkgs/yaml/lib/src/scanner.dart
@@ -653,7 +653,7 @@
     if (_inBlockContext) {
       if (!_simpleKeyAllowed) {
         throw new YamlException(
-            "Block sequence entries are not allowed in this context.",
+            "Block sequence entries are not allowed here.",
             _scanner.emptySpan);
       }
 
@@ -676,7 +676,7 @@
   void _fetchKey() {
     if (_inBlockContext) {
       if (!_simpleKeyAllowed) {
-        throw new YamlException("Mapping keys are not allowed in this context.",
+        throw new YamlException("Mapping keys are not allowed here.",
             _scanner.emptySpan);
       }
 
@@ -714,14 +714,15 @@
       // A simple key cannot follow another simple key.
       _simpleKeyAllowed = false;
     } else if (_inBlockContext) {
-      // If we're here, we've found the ':' indicator following a complex key.
-
       if (!_simpleKeyAllowed) {
         throw new YamlException(
-            "Mapping values are not allowed in this context.",
+            "Mapping values are not allowed here. Did you miss a colon "
+                "earlier?",
             _scanner.emptySpan);
       }
 
+      // If we're here, we've found the ':' indicator following a complex key.
+
       _rollIndent(
           _scanner.column,
           TokenType.BLOCK_MAPPING_START,

pkgs/yaml/pubspec.yaml

diff --git a/pkgs/yaml/pubspec.yaml b/pkgs/yaml/pubspec.yaml
index e8a62c0..d00d31f 100644
--- a/pkgs/yaml/pubspec.yaml
+++ b/pkgs/yaml/pubspec.yaml
@@ -1,5 +1,5 @@
 name: yaml
-version: 2.1.3-dev
+version: 2.1.3
 author: "Dart Team <misc@dartlang.org>"
 homepage: https://github.com/dart-lang/yaml
 description: A parser for YAML.