Add missing error: default values must be type correct

Author: eernstg

specification/dartLangSpec.tex

@@ -2457,17 +2457,9 @@ \subsubsection{Optional and Named Formals}
 
 <defaultNamedParameter> ::= \gnewline{}
   <metadata> \REQUIRED? <normalFormalParameterNoMetadata>
-  \gnewline{} ((`=' | `:') <expression>)?
+  \gnewline{} (`:' <expression>)?
 \end{grammar}
 
-\LMHash{}%
-The form \syntax{<normalFormalParameter> `:' <expression>}
-is equivalent to the form
-\syntax{<normalFormalParameter> `=' <expression>}.
-The colon-syntax is included only for backwards compatibility.
-It is deprecated and will be removed in
-a later version of the language specification.
-
 \LMHash{}%
 It is a \Error{compile-time error} if an optional or named formal parameter has
 the modifier \CONST{} or the modifier \LATE.
@@ -2482,6 +2474,11 @@ \subsubsection{Optional and Named Formals}
 It is a
 \Error{compile-time error} if a required named parameter has a default value.
 
+\LMHash{}%
+Assume that $p$ is an optional named parameter with default value $d$.
+It is a \Error{compile-time error}
+if the static type of $d$ is not assignable to the declared type of $p$.
+
 \LMHash{}%
 It is a \Error{compile-time error} if the name of a named optional parameter
 begins with an `_' character.