Allow surrogates in string literals.

docs/language/dartLangSpec.tex

 \documentclass{article}
 \usepackage{epsfig}
 \usepackage{color}
 \usepackage{dart}
 \usepackage{bnf}
 \usepackage{hyperref}
 \usepackage{lmodern}
 \newcommand{\code}[1]{{\sf #1}}
 \title{Dart Programming Language  Specification  \\
 {4th edition draft}\\
@@ skipping 2772 matching lines @@
 \item  $\backslash$n for newline, equivalent to $\backslash$x0A.
 \item $\backslash$r for carriage return, equivalent to $\backslash$x0D.
 \item $\backslash$f for form feed, equivalent to $\backslash$x0C.
 \item $\backslash$b for backspace, equivalent to $\backslash$x08.
 \item $\backslash$t for tab, equivalent to $\backslash$x09.
 \item $\backslash$v for vertical tab, equivalent to $\backslash$x0B
 \item $\backslash$x $HEX\_DIGIT_1$ $HEX\_DIGIT_2$, equivalent to
 
 $\backslash$u\{$HEX\_DIGIT_1$ $HEX\_DIGIT_2$\}.
 \item $\backslash$u $HEX\_DIGIT_1$ $HEX\_DIGIT_2$ $HEX\_DIGIT_3$ $HEX\_DIGIT_4$, equivalent to $\backslash$u\{$HEX\_DIGIT_1$ $HEX\_DIGIT_2$ $HEX\_DIGIT_3$ $HEX\_DIGIT_4$\}.
-\item $\backslash$u\{$HEX\_DIGIT\_SEQUENCE$\} is the unicode scalar value represented by the $HEX\_DIGIT\_SEQUENCE$. It is a compile-time error if the value of the $HEX\_DIGIT\_SEQUENCE$ is not a valid unicode scalar value.
+\item $\backslash$u\{$HEX\_DIGIT\_SEQUENCE$\} is the Unicode code point represented by the $HEX\_DIGIT\_SEQUENCE$.

[floitsch, 2016/09/15 15:09:19]

It might only be the unicode code unit, since we now allow surrogate pairs.

[Lasse Reichstein Nielsen, 2016/09/15 15:50:37]

The term "code unit" presumes an encoding form (e.g., UTF-16 where code units
are 16-bit unsigned integers). Here we are actually talking about the code point
itself. The values in the surrogate range are code points but not Unicode scalar
values (that's basically the definition of "Unicode scalar value": Code points
except high and low surrogates), hence the change.

I should put back the "compile-time error" part, though, since six hex digits
can specify a value which is not a valid code unit - anything above 0x10FFFF.

 \item \$ indicating the beginning of an interpolated expression.
 \item Otherwise, $\backslash k$ indicates the character $k$ for any $k$ not in $\{n, r, f, b, t, v, x, u\}$.
  \end{itemize}
 
 \LMHash{}
 Any string may be prefixed with the character `r', indicating that it is a {\em raw string}, in which case no escapes or interpolations are recognized.
 
 \LMHash{}
 It is a compile-time error if a non-raw string literal contains a character sequence of the form $\backslash$x that is not followed by a sequence of two hexadecimal digits. It is a compile-time error if a non-raw string literal  contains a character sequence of the form $\backslash$u that is not followed by either a sequence of four hexadecimal digits, or by curly brace delimited sequence of hexadecimal digits.
 
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 The invariant that each normative paragraph is associated with a line
 containing the text \LMHash{} should be maintained.  Extra occurrences
 of \LMHash{} can be added if needed, e.g., in order to make
 individual \item{}s in itemized lists addressable.  Each \LM.. command
 must occur on a separate line.  \LMHash{} must occur immediately
 before the associated paragraph, and \LMLabel must occur immediately
 after the associated \section{}, \subsection{} etc.
 
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