Avoid lexical declarations in case/default clauses

This commit adds guidance warning people to avoid lexical declarations
in case/default clauses and enables the corresponding ESLint rule.

  http://eslint.org/docs/rules/no-case-declarations.html

We didn't have a section on switch statements yet, so I thought about
starting one. It seemed like it would best fit between section 15
(Comparison Operators & Equality) and section 16 (Blocks), but I didn't
want to mess up all of the following numberings, since people probably
have references to them. I considered adding this near the end to
minimize this effect, but it really seemed to belong near these other
things. I landed on appending it to Section 15 (Comparison Operators &
Equality) and I think it sorta fits there since switch statements are a
related concept.

README.md

@@ -1245,6 +1245,47 @@ Other Style Guides
     ```
 
   - [15.4](#15.4) <a name='15.4'></a> For more information see [Truth Equality and JavaScript](http://javascriptweblog.wordpress.com/2011/02/07/truth-equality-and-javascript/#more-2108) by Angus Croll.
+  - [15.5](#15.5) <a name='15.5'></a> Use braces to create blocks in `case` and `default` clauses that contain lexical declarations (e.g. `let`, `const`, `function`, and `class`).
+
+  > Why? Lexical declarations are visible in the entire `switch` block but only get initialized when assigned, which only happens when its `case` is reached. This causes problems when multiple `case` clauses attempt to define the same thing.
+
+  eslint rules: [`no-case-declarations`](http://eslint.org/docs/rules/no-case-declarations.html).
+
+    ```javascript
+    // bad
+    switch (foo) {
+      case 1:
+        let x = 1;
+        break;
+      case 2:
+        const y = 2;
+        break;
+      case 3:
+        function f() {}
+        break;
+      default:
+        class C {}
+    }
+
+    // good
+    switch (foo) {
+      case 1: {
+        let x = 1;
+        break;
+      }
+      case 2: {
+        const y = 2;
+        break;
+      }
+      case 3: {
+        function f() {}
+        break;
+      }
+      default: {
+        class C {}
+      }
+    }
+    ```
 
   - [15.5](#15.5) <a name='15.5'></a> Ternaries should not be nested and generally be single line expressions.
 

packages/eslint-config-airbnb/rules/best-practices.js

@@ -24,6 +24,8 @@ module.exports = {
     'no-alert': 1,
     // disallow use of arguments.caller or arguments.callee
     'no-caller': 2,
+    // disallow lexical declarations in case/default clauses
+    'no-case-declaration': 2,
     // disallow division operators explicitly at beginning of regular expression
     'no-div-regex': 0,
     // disallow else after a return in an if