Model each Dart library as its own ScriptState when devtools are enabled.

LayoutTests/dart/inspector/evaluate-in-console-expected.txt

 CONSOLE MESSAGE: line 1: Hello <window>
 CONSOLE MESSAGE: line 1: foo
 Debugger was enabled.
 
 Running: testScopeChain
 Script execution paused.
 Call stack:
     0) handleMessage (evaluate-in-console.dart:13)
 test.instanceField = "Test.instanceField"
 test.instanceField = "new value" = "new value"
@@ skipping 27 matching lines @@
 test.create("x").instanceField = "x"
 test.concat(test.create("x"), test.create("y")) = "new value:x:y"
 _private = "private"
 globalField = null
 calculateSquareRoot(25) = 5
 new Element.tag("div").outerHtml = "<div></div>"
 (new Element.tag("div")..append(new Element.tag("h1"))).outerHtml = "<div><h1></h1></div>"
 new Element.tag("div").tagName = "DIV"
 intList.toString() = "{4, 2}"
 (LinkedList).toString() = "LinkedList"
-$var.a=new List<int>()..add(4)..add(42) = 
-$var.a.toString() = "[4, 42]"
-$var.variables() = 
-$var.f = (x) => x*42 = [Dart Function] {}
-$var.f(2) = 84
+var a=new List<int>()..add(4)..add(42) = 
+a.toString() = "[4, 42]"
+$consoleVariables.variables() = 
+var f = (x) => x*42 = [Dart Function] {}
+f(2) = 84
 [] is List = true
 {} is List = false
 "This should be an int: ${10000000000000000000000 - 1}" = "This should be an int: 9999999999999999999999"
 10000000000000000000000 is int = true
 1.0 is int = false
 1.0 is double = true
 ((x) => x * 2)(21) = 42
 ((x) { var z = x * 4; return z; })(21) = 84
 print("Hello $window") = Hello <window>
 
 Running: testPrint
 Script execution resumed.
 Debugger was disabled.