Variable name is expected to be in uppercase only

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What steps will reproduce the problem?
1. Define a variable e.g: @def width 10
2. Use this variable in style as .style1 { width: width; }
3. width is not replaced with the value 10.

What is the expected output? What do you see instead?
Expected output is width to be assigned value 10 which instead is being set as 
width.

What version of the product are you using? On what operating system?
closure-stylesheet version: using jar closure-stylesheets-20111230.jar
OS: windows7

Please provide any additional information below.
It is no where documented that variable name must be in upper case. It is 
documented only for mixins. In code, the condition isDefinitionReference of 
CssConstantReferenceNode class is used both for mixins and variable 
verification and only throws error for mixins and not variables. Hence, the 
pareser silently moves forward without replacing the lower case variable values 
and we get the issues at later stages.

Original issue reported on code.google.com by nitu....@gmail.com on 7 Aug 2012 at 10:44