Adding return-from-computed recommended rule. Fixes ember-cli#247.

docs/rules/return-from-computed.md

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+## Always return a value from a computed property function
+
+### Rule name: `return-from-computed`
+
+When using computed properties always return a value.
+
+```javascript
+import Ember from 'ember';
+
+const {
+  Component,
+  computed
+} = Ember;
+
+export default Component.extend({
+  firstName: null,
+  lastName: null,
+
+  // GOOD
+  fullName: computed('firstName', 'lastName', {
+    get(key) {
+      return `${this.get('firstName')} ${this.get('lastName')}`;
+    },
+    set(key, value) {
+      let [firstName, lastName] = value.split(/\s+/);
+      this.set('firstName', firstName);
+      this.set('lastName',  lastName);
+      return value;
+    }
+  }),
+
+  // BAD
+  fullName: computed('firstName', 'lastName', {
+    get(key) {
+      return `${this.get('firstName')} ${this.get('lastName')}`;
+    },
+    set(key, value) {
+      let [firstName, lastName] = value.split(/\s+/);
+      this.set('firstName', firstName);
+      this.set('lastName',  lastName);
+    }
+  })
+});
+```

lib/rules/return-from-computed.js

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+'use strict';
+
+const utils = require('../utils/utils');
+
+//------------------------------------------------------------------------------
+// General rule - Don't introduce side-effects in computed properties
+//------------------------------------------------------------------------------
+
+module.exports = {
+  meta: {
+    docs: {
+      description: 'Warns about missing return statements in computed properties',
+      category: 'Possible Errors',
+      recommended: false,
+      url: 'https://github.com/ember-cli/eslint-plugin-ember/tree/master/docs/rules/no-side-effects.md'
+    },
+    fixable: null, // or "code" or "whitespace"
+  },
+
+  create(context) {
+    const message = 'Return a value from computed property';
+
+    const report = function (node) {
+      context.report(node, message);
+    };
+
+    return {
+      'CallExpression[callee.name="computed"]': function (node) {
+        const objExps = utils.findNodes(node.arguments, 'ObjectExpression');
+        const fnExps = utils.findNodes(node.arguments, 'FunctionExpression');
+
+        if (objExps.length) {
+          const computedObj = objExps[0];
+          const props = computedObj.properties;
+
+          props.forEach((prop) => {
+            if (utils.isFunctionExpression(prop.value) && !utils.findNodes(prop.value.body.body, 'ReturnStatement').length) {
+              report(prop.value);
+            }
+          });
+        }
+
+        if (fnExps.length) {
+          const computedFn = fnExps[0];
+
+          if (!utils.findNodes(computedFn.body.body, 'ReturnStatement').length) {
+            report(computedFn);
+          }
+        }
+      }
+    };
+  }
+};

tests/lib/rules/return-from-computed.js

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+// ------------------------------------------------------------------------------
+// Requirements
+// ------------------------------------------------------------------------------
+
+const rule = require('../../../lib/rules/return-from-computed');
+const RuleTester = require('eslint').RuleTester;
+
+// ------------------------------------------------------------------------------
+// Tests
+// ------------------------------------------------------------------------------
+
+const eslintTester = new RuleTester();
+eslintTester.run('return-from-computed', rule, {
+  valid: [
+    {
+      code: 'let foo = computed("test.length", { get() { return ""; }, set() { return ""; }})',
+      parserOptions: { ecmaVersion: 6, sourceType: 'module' },
+    },
+    {
+      code: 'let foo = computed("test", function() { return ""; })',
+      parserOptions: { ecmaVersion: 6, sourceType: 'module' },
+    }
+  ],
+  invalid: [
+    {
+      code: 'let foo = computed("test", function() { })',
+      parserOptions: { ecmaVersion: 6, sourceType: 'module' },
+      errors: [{
+        message: 'Return a value from computed property'
+      }]
+    },
+    {
+      code: 'let foo = computed("test", function() { if (true) { return ""; } })',
+      parserOptions: { ecmaVersion: 6, sourceType: 'module' },
+      errors: [{
+        message: 'Return a value from computed property'
+      }]
+    },
+    {
+      code: 'let foo = computed("test", { get() {}, set() {} })',
+      parserOptions: { ecmaVersion: 6, sourceType: 'module' },
+      errors: [
+        {
+          message: 'Return a value from computed property'
+        },
+        {
+          message: 'Return a value from computed property'
+        }
+      ]
+    },
+    {
+      code: 'let foo = computed("test", function() { })',
+      parserOptions: { ecmaVersion: 6, sourceType: 'module' },
+      errors: [{
+        message: 'Return a value from computed property'
+      }]
+    }
+  ],
+});